I think trying to appear as if it’s the worst day is a really terrible idea. If it happens that it’s not your worst day for symptoms, but you are trying to ‘act’ as if it is, you run the risk of looking like a total fraud. Obviously if it is a bad day, then let it show.
Obviously, don’t go to efforts to do things that you normally couldn’t. For example, if you’ve said in your claim that you can’t do up shoe laces and buttons (and you don’t have a carer who helps you dress), don’t go wearing lace up shoes and a button up shirt. If you need to use a walker, a wheelchair or other aid to walk, use it. If it takes you a long time to walk a given distance then make sure you walk / move at that speed. If you need a rest, then take it. If you have to take a taxi to the assessment centre (because of difficulties getting there at other way), make sure they know this. If you need help with personal care, make sure they know.
If you have any written evidence that you’ve not previously submitted, take it and see if you can get it included in their decision making.
Make sure you have company with you (if that’s possible).